Apparatus for extruding plastic material



July 7, 1959 Filed May 8, 1956 G. E. HENNING APPARATUS FOR EXTRUDINGPLASTIC MATERIAL 2 Sheets-Sheet l INVENTOR.

6. E. HENN/NG BY ATTORNEY United States Patent 0 George E. Henning,Baltimore, rjMd., assignor to Western Electric Company, Incorporated,New York, N.Y., a

'Thisinvention relates to apparatus for extruding plastic material,=andm6re particularly to apparatus for extruding organic insulating andjacketing material pon continuously-adyancingfilamentary conductors or Ocl c In producing an extruded coveringvqn a filamentary conductor,orftheflike, serious difficulties often'have been encounteredheretofore'inmaintaining the covering concentric :with respect to theconductor. One of the principal cau ses of eccentricity is the presenceof unbalanced flow conditions in the extrusion head employed to form athe covering on the 4 conductor. This proble'rn of v unbalrahhel aw w'ti hs inheren i a f s t 6 t i d o t wi n hhe t ph- The extrusion head of'a conventional extruder ,of the cross head typeis provided generally,with anextrusion passage that I communicates with and extendstransversefly across, the exit endjof the extrusion, cylinder in which astock screw f iS p0Sitioned for forcing Ian extrudable plasticmaterialinto the ,extrusionp assage. A core I guide is located atone,end off'the transversely extending extrusion passa anditheiconductoritojbe insulated is ad- [tam t o ha g'u dra'ssa i t o l d intothe 1 extrusibn'passage at a point closely adjacent to a forming diepositioned in the exit end of, the extrus ionpassage. The core,guidecenters the conductor so thatfit tends to; pass through the axialcenter of the forming di el i ,The plastic material enyelops .theconductor in the ,spacepbetweenthetwall of the extrusion die orifice and,{theflexit ,end of l the core guide andfis formed into a tubularcovering surrounding ,the continuously advancing conductor. To producefaconcentric covering onlthe conductor, it is essential that, the quantityof. flow, of the ,plastic ,material'belbalanced, throughout the entirecross vo se ctio nof the extrusion passage/in the vicinity'in which theinertia almenvelops theco nductdr. 'The problem of maintainingalbalance'd flowcondition is complicated by thefact thatfinthefcrossihead 'typefextruderthere is an abrupt, nge in'lthesdirectionor new or the plastic material betw enthe stockisjc rew and thedieorifice.

w A ida blef9 0 'bendathe length of the he plastic material flowingthrough ross,;section of extrusion pasack; screw is substantially lshorter 1 a raverse by; the-mat i rflo iha eu t portiond'ametricallyiopposite. ;.;I n addition, insome ex- ,atrud r 10 thi t puh iflQ 9 h Pl t material thr oughg the portion. of thercross sectioniofthe extrusion passagenearestrto:the stock screw approximates that ofshow; houn e .1 by {a a n-a p wh in the ametr a yaoppo iteipottion oftth s section the afiowsappwx mates that totea fl i flow \bounded'by woarallel; plates.

-Hence,;;the; friction losses in, the portion l f-aicrossisectionmf:tlie-xxtrrrsionpassage nearest to the ockascrewmre appreciably 4lowerithan ythoserels'ewhere .1,mttheacros-stsection; i

As a result of the differencesinthe lengths eofrithe 2,893,056 :PatentedJuly 7, 1959 paths traversed by the plastic rnaterialinlvariousvzportions of the extrusion passage and 1 the physical,environment of theirassociated flows, there exist substantial ;dif-.ferences in the consistency and rate of fiowthroughout a cross sectionof the ,fiow ofplastic material entering the die,orifice, the .portionof the cross jsection near'est to vthe stock screw havingthehighest rate.of lsfloiw and the more remote,portionpwhichis diametrically oppositethereto having ,the lowest rate ,of flow. The'ztresultant unbalance offlows through the cross section of i'ftheieir- ,t'rusion passageat.the.;entranc elto the dieiorifice' gives 'rise'to eccentricity in.ithe covering "extruded Iuponathe conducton'the thinnest portion of,the' eccentric covering occurring in'that portion of the insulationproduced in the'region where the lowest rate of flow exists.

Various apparatus have been devised heretofore in an attempt'to obtainbalanced flow conditions in extruders to insure concentric'ity of theextruded'covering ona finished insulated conductor. Heretofore thesolutions to this problem have not been entirely satisfactory'or'requirerelatively expensivechanges inthe design of conventional extrudingapparatus.

It is an objecfof this invention to provide new and improved apparatusfor extruding plastic materiah Another object of this invention is toprovide new and improved apparatus for extrudingorganic plasticinsulating and jacketing material upon continuously aavancing'filamentary conductors or other cores.

Apparatus illustrating certain features of the invention may include animprovement in extrusion apparatus comprising an extrusion cylinderhaving an ext-fusion bore formed therein, an extrusion head mountedat'the discharge end of the extrusion cylinder'and provided with apassageway communicating with the extrusion bore and extendingtransversely'with respect'th'ere tofineans ,for forcing a plasticmaterial along the extrusion ib'ore into the passageway, andan'exti'usion die mounted at one end of the passageway.' The improvern'eliit'frnay comprise means for directing the plastic material ern'erging from the extrusion bore initially toward the' end of thepassageway remote from the extrusion die, auditor directing the plasticmaterial'subsequently from the re 'moteend of *the passa'geway to theextrusion die. A complete understanding of the invention'may beobtained'from the following detailed descriptionjofapparatus formingspecific embodiments thereof, whenre'ad in conjunction with the appendeddrawingsjin Fig. l'is" a fragmentary, horizontalsection of extrudingapparatus illustrating certain ffeaturesfof the infve ntionj v Fig. 2 isa fragmentary, vertical section taken along line 22 of Fig. l; i Fig. 3is a fragmentary, vertical section talgen along line 3--3 of Fig. l and*Fig. 41s a side ,view of a toolholder'forming apart 7 of the apparatusshown in Fig. 1.

Referring now in detail to the drawings, there is shown in Fig. Ianextrusion cylinder 10 having formedthere- "in an elongated, cylindricalextrusion bore 11"in whi'ch,a ew 1 2 designed to 'be rotated by suitabledrive ,rneans (not shown) for the purpose of working and gforcingifaplastioinsulating material 14 along the extrusion bore 11 and into andthrough a tapered opening 16,infa bod y member 18 forming part of anextrusion head, designated generally by the numeral 20.

Theextrusion head 20 is mounted detachably to the discharge end of theextrusion cylinder =1'osa thatjthe tapered opening '16 forms acontinuation of the'extrusion bore 11. Po'sitioned transverselyacrossth'e tapered opening 16 in the extrusion head '20- in a"per fc''ra e'd, fnustoconicalw; backing plate 24 upon which issupportcd ascreen pack 25. The tapered opening 16 communicates with a cylindricalbore 28 formed in the body member 18 transversely with respect to thelongitudinal axis of the extrusion bore 11.

I? A generally cylindrical tool holder, designated generally by thenumeral .30, having a cylindrical bore 32 formed coaxially therein ismounted removably within the body member 18 and is held in place bymeans of a threaded retainer nut 34. The retainer nut 34 is engagedthreadedly within a complementary, threaded counterbore 35 formed in thebody member 18. The tool holder 30 is provided additionally with acoaxial counterbore 37.

An annular die holder, designated generally by the numeral 38, ismounted threadedly within a threaded portion 39 at the lefthand end, asviewed in Fig. 1, of the counterbore 37. A reduced cylindrical endportion 40 of the die holder 38 fits closely within and extends partlyinto the counterbore 37 An extrusion die 45 is positioned coaxiallywithin a complementary die-receiving seat 47 provided in the die holder38. The die 45 is oriented so that the longitudinal axis of an extrusionorifice 50 formed therein is aligned accurately on the commonlongitudinal axis of the counterbore 37 and the bore 32 formed in thetool holder 30.

A hollow core guide, designated generally by the numeral 52, is mountedwithin the counterbore 37 and has a reduced, cylindrical shank portion54 extending through the bore 32, and through a central aperture 55provided in the retainer nut 34. The core guide 52 is positionedconcentrically with respect to the counterbore 37 and has afrustoconical neck 56, which tapers slightly from a maximum diameter atthe righthand end of the counterbore 37 to a minimum diameter adjacentto the extrusion die 45. Formed integrally with the lefthand end of theneck 56 is an enlarged, generally frustoconical, pluglike head 60 whichfits closely within a complementary, frustoconical mouth 62 of theextrusion orifice S0. The head 60 is tapered at an included angle ofapproximately 60 and has a maximum diameter at the righthand end thereofand a minimum diameter at the lefthand end thereof.

The frustoconical periphery of the head 60 is relieved by a plurality ofsubstantially identical radial grooves 65-65 spaced equally therearound.The grooves 6565 connect the die orifice 50 with a relatively large,annular, pressure-equalizing chamber 67 formed by the space between theneck 56 of the core guide 52 and the cylindrical wall of the counterbore37. The pressure-equalizing chamber is bounded at one end by the head 60and at the other end by the base at the righthand end of the counterbore37.

From the mouth 62 the throat die orifice tapers frustoconically at anincluded angle of approximately 20 with respect to the longitudinal axisthereto from a maximum diameter at the righthand end thereof to aminimum diameter at the lefthand end thereof. The core guide 52 isprovided with a longitudinally extending passage 69 aligned accuratelywith the longitudinal axis of the die orifice 50 to guide a conductor 70advancing through the passage longitudinally through the center of thedie orifice.

The tool holder 30 is provided with two identical, interconnected,generally L-shaped channels 71 and 72 *(Figs. 2, 3 and 4) formed in thecylindrical surface thereof. The channels 71 and 72 extend arcuately inopposite directions from the exit end of the tapered opening 16 in thebody member 18 to diametrically opposite positions at the center of thetool holder 30. At this point they turn through 90 angles and extendlongitudinally along the tool holder toward the righthand end of thecounterbore 37, as viewed in Fig. 1.

The channels 71 and 72 communicate with relatively small, radiallyextending feed ports 73 and 74, respectively, formed in the wall of thetool holder. The ports 73 and 74 communicate, in turn, with thepressure-equalizing chamber 67 near the extreme righthand end thereof,as viewed in Fig. 1. The feed ports 73 and 74 are identical in size andshape and preferably are circular in cross section. The feed ports 73and 74 are positioned diametrically opposite to each other andsymmetrically with respect to a horizontal plane containing thelongitudinal axis of the tool holder 30, whereby they are equidistantlyspaced from the exit end of the tapered opening 16 in the body member18.

Operation During the operation of the extruding apparatus thefilamentary conductor 70 is advanced longitudinally at a predeterminedspeed in the direction of the arrow shown in Fig. l. At the same time asuitable plastic material 14, such as polyethylene or the like, isadvanced by the rotating stock screw 12 through the extrusion bore 11and into the tapered opening 16 in the extrusion head 20.

Upon leaving the tapered opening 16, the plastic material 14 is splitinto two streams and flows through the channels 71 and 72. The twoseparate streams of plastic material 14 initially flow circumferentiallyin opposite directions and then rearwardly along the channels 71 and 72in the tool holder 30 to the feed ports 73 and 74, respectively. The twoseparate streams then flow through the ports 73 and 74, and enter thepressure-equalizing chamber 67 near the extreme righthand end thereof.

Since the channels 71 and 72 are identical and feed ports 73 and 74 arespaced equidistantly from the exit end of the tapered opening 16, thepaths traversed by the two separate streams of plastic material 14 areidentical in all respects. Accordingly, the quantities of plasticmaterial entering the pressure-equalizing chamber 67 through the two,identical feed ports 73 and 74 are equal.

Due to the fact that the pressure-equalizing chamber 67 has a relativelylarge volumetric capacity, disturbances created by the plastic material14 entering the chamber through the feed ports 73 and 74 are effectivelydamped out. The feed ports 73 and 74 are located near the extremerighthand end of the pressure-equalizing chamber 67 and, hence, suchdisturbances are confined to the portion of the chamber which isrelatively remote from the die orifice 50.

In addition, as a result of the relatively large cross sectional area ofthe pressure equalizing chamber 67, the velocity of the flow of plasticmaterial 14 within the chamber 67 is relatively low compared to the flowof plastic material through the throat of the die orifice 50 at itsminimum diameter, This relationship acts to main tain what is virtuallya constant static pressure at the entrance to the die orifice 50 and theplastic material flowing into the die orifice through the radial grooves-65 is substantially uniform in consistency and rate of flow throughout.Thus, the quantity of plastic material flowing throughout a crosssection of the die orifice 50 is completely balanced and results in theapplication of a concentric, tubular covering 80 on the conductor 70,which covering is uniform in wall thickness throughout.

In the selection of the size and location of the feed ports 73 and 74care must be taken to insure that the feed ports are locatedsufiiciently far enough away from the mouth 62 of the die orifice 50 toinsure the damping out of any disturbances resulting from the flow ofplastic material 14 as it passes through the feed ports 73 and 74 fromthe channels 71 and 72, respectively, into the pressure-equalizingchamber 67. In addition, to minimize the pressure drop through the feedports 73 and 74, the combined cross sectional area of the feed portsshould be preferably slightly larger than the free area surrounding theconductor at the minimum diameter of the throat of the die orifice 50.On the other hand, the cross sectional area of each of the feed ports 73and 74 must be held relatively small in order to minimize any tendencyfor the plastic material to flow along the path of the least resistance,which is adjacent to the sides of the feed ports nearest o. the stockscrew 12,.

In a specific apparatus embodying the principles of the inventionsubstantially balanced flow conditions were obtained by properlydimensioning the chamber 67, the channels 71 and 72 and the feed ports73 and 74, to achieve the following relationships between the averagevelocities of the flow of the plastic material 14 at various points:

where:

V =Average velocity of the plastic material flowing through the freearea of the die orific 50;

V =Average velocity of the plastic material flowing through thepressure-equalizing chamber 67;

V =Average velocity of the plastic material flowing through the feedports 73 and 74, and

V =Average velocity of the plastic material flowing through the channels71 and 72.

It will be understood that the term plastic material" as employed in thespecification and appended claims is meant to include thermoplasticmaterials, such as poly ethylene, polyvinyl chloride compounds, and thelike, and also thermosetting materials, such as rubber compounds,neoprene compounds, and the like.

It is manifest that the above-described embodiment of the invention ismerely illustrative of an application of the principles of the inventionand that numerous other arrangements may be readily devised by thoseskilled in the art incorporating the broad principles of the invention.

What is claimed is:

In an apparatus for extruding plastic material including an extrusioncylinder and an extrusion head having a coaxial passageway thereinmounted transversely with respect to the extrusion cylinder, theimprovement which comprises a substantially cylindrical tool holdermounted closely, and detachably in the passageway in the head and havinga coaxial bore and a coaxial counterbore, a portion of the counterboreforms a relatively large, hollow pressure-equalizing chamber extendingcoaxially with respect to the head and transversely on opposite sides ofthe cylinder, a channel formed by a peripheral groove extendingsubstantially one hundred and eighty degrees circumferentially of thetool holder and positioned so that the peripheral groove is symmetricalwith respect to the axis of the cylinder, a pair of channels formed by apair of parallel, longitudinally extending peripheral grooves extendingrearwardly from the ends of the first-mentioned channel in diametricallyopposite portions of the tool holders, a pair of relatively small,radial feed ports positioned at diametrically opposite portions of thetool holder adjacent to the end of the chamber and connecting thechamber with the rearwardly extending channels, a die holder mounted inthe forward end of the counterbore adjacent to the hollow chamber, a dieprovided with a frustoconical bore and positioned in the die holder, acore guide having a grooved, frustoconical, plug-like head fittingwithin a complementary portion of the bore in the die, means foradvancing successive portions of a core of indefinite lengthcontinuously through the core guide and die, and means for forcingplastic material through the cylinder and head and out the free areabetween the core and die, the sizes of each of the channels forming thepair of channels, each of the feed ports and the pressureequalizingchamber being such that the average velocity of the plastic materialflowing therethrough will be re-- spectively approximately 0.05, 0.88,and 0.036 times the average velocity of the plastic material flowingthrough the minimum free area between the core and die orifice-References Cited in the file of this patent UNITED STATES PATENTS2,766,480 Henning Oct. 16, 1956 2,766,481 Henning Oct. 16, 1956 FOREIGNPATENTS 624,699 Great Britain June 15, 1949 649,873 Great Britain Feb.7, 1951

